The immune response to polysaccharide antigens is highly regulated and has several distinguishing features. Among them are restricted use of immunoglobulin subclasses, restricted use of immunoglobulin variable regions and late development in ontogeny. The basis for selective VH gene usage in response to antigens such as bacterial levan (BL) is poorly understood. BL is a b-(2--6) polyfructosan with b-(2--1) branch points. We have generated a panel of 27 monoclonal antibodies (mAb) from CBA/CaHN mice following one or two doses of 10 mg BL. Twenty-five of 27 mAb were specific for BL and 1 cross-reacted with inulin a b-(2-- 1) poly-fructosan. The primary mAb all have m heavy chains and k light chains. The secondary mAb have either a m or g 3 heavy chains and 14 of 16 have k light chains and 2/16 have l1 light chains. VH gene family analysis showed a significant over- representation of the 36-60 gene family which was expressed in 37% of the mAb compared to the expected frequency of 4% (p less than 0.0001). The majority of mAb (48%) used the J558 family as expected. Two BALB/c myeloma proteins specific for BL use in a single gene of the X24 VH gene family. Because BALB/c and CBA mice are closely related at the immunoglobulin locus, it was surprising that none of the 27 CBA mAb used the X24 VH gene family. We, therefore, have begun to further examine VH gene usage in mAb derived from BALB/c mice imunized with BL. A panel of 30 BALB/c mAb, has been generated, 25 primary and 5 secondary. These mAb used either m or g 3 heavy chains and 29/30 used k light chains. In contrast to CBA mAb, most of the BALB/c (18/30) cross-reacted with inulin. The VH genes have been examined in the group of secondary mAb and the X24 gene was found in 1 of 5 mAb. Two of five used J558 and 2/5 used the 7183 gene family. The primary mAb are currently being studied. We conclude that CBA and BALB/c antibodies specific for BL have different fine specificities and have different VH gene family usage.